After migration, the permissions the guest device wants to impose on its
BlockBackend are stored in blk->perm and blk->shared_perm.  In
blk_root_activate(), we take our permissions, but keep all shared
permissions open by calling `blk_set_perm(blk->perm, BLK_PERM_ALL)`.

Only afterwards (immediately or later, depending on the runstate) do we
restrict the shared permissions by calling
`blk_set_perm(blk->perm, blk->shared_perm)`.  Unfortunately, our first
call with shared_perm=BLK_PERM_ALL has overwritten blk->shared_perm to
be BLK_PERM_ALL, so this is a no-op and the set of shared permissions is
not restricted.

Fix this bug by saving the set of shared permissions before invoking
blk_set_perm() with BLK_PERM_ALL and restoring it afterwards.

Fixes: 5f7772c4d0cf32f4e779fcd5a69ae4dae24aeebf
       ("block-backend: Defer shared_perm tightening migration
       completion")
Reported-by: Peng Liang <liangpeng10@huawei.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
---
 block/block-backend.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/block/block-backend.c b/block/block-backend.c
index 12ef80ea17..41e388fe1f 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -190,6 +190,7 @@ static void blk_root_activate(BdrvChild *child, Error **errp)
 {
     BlockBackend *blk = child->opaque;
     Error *local_err = NULL;
+    uint64_t saved_shared_perm;
 
     if (!blk->disable_perm) {
         return;
@@ -197,12 +198,22 @@ static void blk_root_activate(BdrvChild *child, Error **errp)
 
     blk->disable_perm = false;
 
+    /*
+     * blk->shared_perm contains the permissions we want to share once
+     * migration is really completely done.  For now, we need to share
+     * all; but we also need to retain blk->shared_perm, which is
+     * overwritten by a successful blk_set_perm() call.  Save it and
+     * restore it below.
+     */
+    saved_shared_perm = blk->shared_perm;
+
     blk_set_perm(blk, blk->perm, BLK_PERM_ALL, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         blk->disable_perm = true;
         return;
     }
+    blk->shared_perm = saved_shared_perm;
 
     if (runstate_check(RUN_STATE_INMIGRATE)) {
         /* Activation can happen when migration process is still active, for
-- 
2.33.1

On 11/25/21 14:53, Hanna Reitz wrote:
> After migration, the permissions the guest device wants to impose on its
> BlockBackend are stored in blk->perm and blk->shared_perm.  In
> blk_root_activate(), we take our permissions, but keep all shared
> permissions open by calling `blk_set_perm(blk->perm, BLK_PERM_ALL)`.
> 
> Only afterwards (immediately or later, depending on the runstate) do we
> restrict the shared permissions by calling
> `blk_set_perm(blk->perm, blk->shared_perm)`.  Unfortunately, our first
> call with shared_perm=BLK_PERM_ALL has overwritten blk->shared_perm to
> be BLK_PERM_ALL, so this is a no-op and the set of shared permissions is
> not restricted.
> 
> Fix this bug by saving the set of shared permissions before invoking
> blk_set_perm() with BLK_PERM_ALL and restoring it afterwards.
> 
> Fixes: 5f7772c4d0cf32f4e779fcd5a69ae4dae24aeebf
>        ("block-backend: Defer shared_perm tightening migration
>        completion")
> Reported-by: Peng Liang <liangpeng10@huawei.com>
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
>  block/block-backend.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

On 11/25/2021 9:53 PM, Hanna Reitz wrote:
> After migration, the permissions the guest device wants to impose on its
> BlockBackend are stored in blk->perm and blk->shared_perm.  In
> blk_root_activate(), we take our permissions, but keep all shared
> permissions open by calling `blk_set_perm(blk->perm, BLK_PERM_ALL)`.
> 
> Only afterwards (immediately or later, depending on the runstate) do we
> restrict the shared permissions by calling
> `blk_set_perm(blk->perm, blk->shared_perm)`.  Unfortunately, our first
> call with shared_perm=BLK_PERM_ALL has overwritten blk->shared_perm to
> be BLK_PERM_ALL, so this is a no-op and the set of shared permissions is
> not restricted.
> 
> Fix this bug by saving the set of shared permissions before invoking
> blk_set_perm() with BLK_PERM_ALL and restoring it afterwards.
> 
> Fixes: 5f7772c4d0cf32f4e779fcd5a69ae4dae24aeebf
>        ("block-backend: Defer shared_perm tightening migration
>        completion")
> Reported-by: Peng Liang <liangpeng10@huawei.com>
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
>  block/block-backend.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 

Thanks for your patch!

Tested-by: Peng Liang <liangpeng10@huawei.com>